Ferroelectric liquid crystal composition

ABSTRACT

A ferroelectric liquid crystal composition is provided which includes at least one achiral compound represented by formula (I) ##STR1## wherein each of R 1  and R 2  is an alkyl group having 1 to 18 carbon atoms, at least one optically active compound represented by formula (II) ##STR2## wherein R 3  is an alkyl group or an alkoxy group having 1 to 18 carbon atoms, R 4  is an alkyl group having 2 to 18 carbon atoms, X is a single bond, --COO--, --OCO--, --N═CH--, --CH═N--, --OCH 2  -- or --CH 2  O--, each of m and n is an integer of 1 or 2, and the symbol * is an asymmetric carbon atom, and at least one compound represented by formula (III) ##STR3## wherein each of ##STR4## R 5  is an alkyl group or an alkoxy group having 1 to 18 carbon atoms, Y is a single bond, --CH 2  O-- or --OCH 2  --, p is an integer of 0 or 1, and Z is an alkyl group having 1 to 18 carbon atoms or --C*H(CH 3 )--O--R 6 , wherein R 6  is an alkyl group having 1 to 18 carbon atoms and the symbol * represents an asymmetric carbon atoms.

TECHNICAL FIELD

The present invention relates to a ferroelectric liquid crystalmaterial. More specifically, it relates to a ferroelectric liquidcrystal composition having quick response properties which comprises asmectic liquid crystal compound and a optically active compound, and anlight switching element in which the above-mentioned composition iscontained.

BACKGROUND TECHNIQUE

Liquid crystal compounds are widely used as materials for displayelements, and most of these liquid crystal elements are based on a TNtype display system and liquid crystal materials are in the state ofnematic phase.

The TN type display system has advantages such as less tiredness of theeyes and extremely small consumption of electric power because of beingof the non-emissive type, whereas it has disadvantages such as slowresponse and disappearance of display at certain visual angles. Inrecent years, this system is being improved in such a direction as tokeep up characteristics of flat displays, and in particular, quickresponse and the enlargement of the visual angle are demanded.

In order to meet these demands, improvements in liquid crystal materialshave been attempted. However, as compared with other emissive typedisplays (e.g., electroluminescence displays and plasma displays), it isapparent that the TN type display system is much poorer with regard toresponse time and the extent of the visual angle.

In order that characteristics of the liquid display element such asfeatures of the non-emissive type and the small consumption of electricpower may be maintained and in order that the quick responsecorresponding to that of the emissive type displays may be assured, itis essential to develop a novel liquid display system in place of the TNtype display system.

As one of such attempts, a display system in which the light switchingphenomenon of ferroelectric liquid crystals is utilized has beensuggested by N. A. Clark and S. T. Lagerwall (see Appl. Phys. Lett. 36,p 899, 1980).

The presence of the ferroelectric liquid crystals was announced for thefirst time in 1975 by R. B. Mayer et al. (see J. Phys., 36, p 69, 1975),and from the viewpoint of a liquid crystal structure, these crystalsbelong to a chiral smectic C phase, a chiral smectic I phase, a chiralsmectic F phase, a chiral smectic G phase and a chiral smectic H phase(hereinafter referred to simply as "S_(C) * phase", "S_(I) * phase","S_(F) * phase", "S_(G) * phase" and "S_(H) * phase", respectively).

In the chiral smectic phase, molecules form a layer and tilt to thesurface of the layer, and a helical axis is vertical to this layersurface. In the chiral smectic phase, spontaneous polarization takesplace, and therefore, when a DC electric field is applied to this layerin parallel therewith, the molecules rotate about the helical axis inaccordance with their polarity. Display elements containing theferroelectric liquid crystals utilize this switching phenomenon.

Nowadays, of the chiral smectic phases, much attention is particularlypaid to the S_(C) * phase. As the display systems in which switchingphenomenon of the S_(C) * phase is utilized, there is a birefringencetype system using two polarizers and a guest/host type system using adichoric dye. Features of these display systems are as follows:

(1) The response is very quick.

(2) Memory properties are present.

(3) Display performance is not quite so affected by the visual angle.

Thus, the display systems have the possibility of achieving thehigh-density display and are considered to be effectively utilizable inthe display elements.

However, in the case that this ferroelectric liquid crystal is used inthe practical display elements, there are now many problems such as slowresponse properties and very difficult alignment.

Therefore, requirements which are necessary for the ferroelectric liquidcrystal materials are

(1) further improvement in the response properties,

(2) the exhibition of the S_(C) * phase in a wide temperature rangeinclusive of room temperatures, and

(3) improvement in alignment.

At present, no chiral smectic liquid crystal compounds in a single statewhich satisfy all of such requirements are present, and thus one attemptis to provide a ferroelectric liquid crystal composition satisfying theabove-mentioned requirements by mixing several chiral smectic liquidcrystal compounds or several non-liquid crystal compounds which can meeta part of the above-mentioned requirements.

In addition to the ferroelectric liquid crystal compositions eachcomprising the ferroelectric liquid crystal compound alone, JapanesePatent Laid-open Publication No. 195,187/1986 discloses a ferroelectricliquid crystal composition which can be prepared by mixing one or morecompounds assuming a ferroelectric liquid crystal phase with fundamentalmaterials of compounds and compositions assuming achiral smectic C, F,G, H and I phases (hereinafter referred to simply as "S_(C) and otherphases").

Furthermore, another ferroelectric liquid crystal composition is alsoreported in which one or more compounds having optical activity but notassuming any ferroelectric liquid crystal phase are mixed withfundamental materials of compounds and compositions assuming the S_(C)and other phases (Mol. Cryst. Liq. Cryst., 89, p 327, 1982).

The above-mentioned smectic liquid crystal mixture which comprises thefundamental material assuming at least one of the achiral S_(C) andother phases and which has at least one of the S_(C) and other phaseswill be hereinafter referred to as the base Sm mixture.

The preferable base Sm mixture is a liquid crystal mixture assuming theS_(C) phase in an extensive temperature range inclusive of roomtemperatures. Components for the base Sm mixture are suitably selectedfrom liquid crystal compounds such as phenylbenzole series, Schiff baseseries, phenylpyridine series and 5-alkyl-2-(4-alkoxyphenyl)pyridine.

For example, in Japanese Patent Laid-open Publication No. 291,679/1986and the pamphlet of PCT International Publication WO86/06401, theferroelectric liquid crystal prepared by mixing5-alkyl-2-(4-alkoxyphenyl)pyrimidine with an optically active compoundcan assume the S_(C) * phase in a wide temperature range inclusive ofroom temperatures. In the former publication, it is also described thatwhen a light switching element is prepared by the use of a ferroelectricsmectic liquid crystal material in which the above-mentioned pyrimidinederivative is used as the base Sm mixture, the thus prepared switchingelement has a shortened response time. Furthermore, in Japanese PatentLaid-open Publication No. 291,679/1986, it is disclosed that theferroelectric liquid crystal material comprising5-alkyl-2-(4'-alkylbiphenylyl-4)pyrimidine,5-alkyl-2-(4-alkoxyphenyl)pyrimidine and an optically active compoundalso assumes the S_(C) * phase in an extensive temperature rangeincluding room temperatures, and that it is effective for theimprovement in the response time. Nevertheless, improvement in theresponse properties at room temperatures is further demanded.

A first object of the present invention is to provide a ferroelectricliquid crystal composition having quick response properties, and asecond object thereof is to provide a switching element containing theliquid crystal composition which has quick response properties.

DISCLOSURE OF THE INVENTION

A ferroelectric liquid crystal composition which is a first feature ofthe present invention comprises at least one of achiral compoundsrepresented by the formula (I) ##STR5## wherein each of R¹ and R² is analkyl group having 1 to 18 carbon atoms, and at least one of opticallyactive compounds represented by the general formula (II) ##STR6##wherein R³ is an alkyl group or an alkoxy group having 1 to 18 carbonatoms, R⁴ is an alkyl group having 2 to 18 carbon atoms, X is a singlebond, --COO--, --OCO--, --N═CH--, --CH═N--, --OCH₂ -- or --CH₂ O--, eachof m and n is an integer of 1 or 2, and the symbol * is an asymmetriccarbon atom, and it is preferred that the amount of the compoundrepresented by the formula (I) is from 20 to 98% by weight, and that ofthe compound represented by the formula (II) is from 1 to 40% by weight.

Furthermore, another ferroelectric liquid crystal composition of thepresent invention comprises a blend of the achiral compound representedby the formula (I) and the optically active compound represented by theformula (II) with a compound represented by the formula (III) ##STR7##wherein each of ##STR8## R⁵ is a alkyl group or an alkoxy group having 1to 18 carbon atoms, Y is a single bond, --CH₂ O-- or --OCH₂ --, p is aninteger of 0 or 1, and Z is an alkyl group having 1 to 18 carbon atomsor --C*H(CH₃)--O--R⁶ wherein R⁶ is an alkyl group having 1 to 18 carbonatoms and the symbol * represents an asymmetric carbon atom, and it ispreferred that the amount of the compound represented by the formula (I)is from 20 to 98% by weight, that of the compound represented by theformula (II) is from 1 to 40% by weight, and that of the compoundrepresented by the formula (III) is from 1 to 40% by weight.

A second invention of the present invention is directed to a lightswitching element containing a ferroelectric liquid crystal compositionwhich contains at least one kind of achiral compounds represented by theformula (I) and at least one kind of optically active compoundrepresented by the formula (II).

Another embodiment of the present case is directed to an light switchingelement which contains a compound represented by the formula (I), acompound represented by the formula (II) and a compound represented bythe formula (III).

Examples of the optically active compounds represented by the formula(II) of the present invention include compounds having the followingformulae ##STR9## wherein R³ is an alkyl group or an alkoxy group having1 to 18 carbon atoms, and R⁴ is an alkyl group having 2 to 18 carbonatoms.

Typical preferable examples of the optically active compoundsrepresented by the formula (II) include ##STR10## which are disclosed inJapanese Patent Laid-open Publication No. 118744/1984 andFerroelectrics, 58, 21 (1984). In addition, similar compounds aredescribed in Japanese Patent Laid-open No. 149547/1985.

The optically active compounds represented by the formula (III) of thepresent invention include compounds represented by the followingformulae ##STR11## wherein R⁵ is an alkyl group or an alkoxy grouphaving 1 to 18 carbon atoms, and R⁶ is an alkyl group having 1 to 18carbon atoms.

Typical examples of the optically active compounds represented by theformula (III) are as follows: ##STR12## and4'-heptyl-4-[4-{2-(butanoyloxy)-propoxy}-phenoxymethyl]-biphenyl and4'-nonyl-4-[4-{2-(pentanoyloxy)-propoxy}-phenoxymethyl]-biphenyl.

Moreover, other compounds can be added to the liquid crystal compositionof the present invention, so long as they do not disturb theaccomplishment of the objects of the present invention.

As the achiral compound represented by the formula (I) of the presentinvention, a material having a smectic C phase is particularlypreferable, but a material which does not exhibit the smectic C phasecan also be used, though its amount is limited to such a range that thetemperature range of the S_(c) phase is not particularly reduced.

Main compounds of such a kind are exemplified in Table 1.

The compounds (I) shown in Table 1 are represented by the formula (I)##STR13## wherein R¹ =n-C_(x) H_(2x+1) and R² =n-C_(y) H_(2y+1).

With regard to phase states, Cr indicates a crystal state, S₃ an unknownsmectic phase, S_(C) a smectic C phase, S_(A) a smectic A phase, N anematic phase, and I_(SO) an isotropic liquid phase. The symbols • and -indicate the presence and the lack of each phase, respectively.

                  TABLE 1 (I)                                                     ______________________________________                                                    Phase Transition                                                  Compound (I)                                                                              Series and its Transition Temperature                             X in R.sup.1                                                                         Y in R.sup.2                                                                           Cr     S.sub.3                                                                             S.sub.C                                                                             S.sub.A                                                                            N     I.sub.SO                        ______________________________________                                        6      6        •47.0                                                                          --    --    •76.7                                                                        --    •                         6      7        •32.5                                                                          --    •50.6                                                                         •76.6                                                                        --    •                         6      8        •27.0                                                                          --    •67.5                                                                         •84.2                                                                        --    •                         6      9        •47.7                                                                          --    •77.2                                                                         •83.6                                                                        --    •                         6      10       •38.1                                                                          •(35.2)                                                                       •82.4                                                                         •86.5                                                                        --    •                         6      11       •38.8                                                                          •42.3                                                                         •84.3                                                                         •86.4                                                                        --    •                         6      12       •35.0                                                                          •47.4                                                                         •85.6                                                                         •87.1                                                                        --    •                         6      13       •41.7                                                                          •48.0                                                                         •82.7                                                                         •84.8                                                                        --    •                         6      14       •34.4                                                                          •54.9                                                                         •85.2                                                                         •86.6                                                                        --    •                         6      15       •49.9                                                                          •56.7                                                                         •83.3                                                                         •85.2                                                                        --    •                         7      5        •44.3                                                                          --    --    •62.0                                                                        •64.2                                                                         •                         7      6        •49.1                                                                          --    --    •77.1                                                                        --    •                         7      7        •32.2                                                                          --    •45.0                                                                         •77.5                                                                        --    •                         7      8        •30.8                                                                          --    •64.0                                                                         •84.4                                                                        --    •                         7      9        •34.2                                                                          --    •76.3                                                                         •85.1                                                                        --    •                         7      10       •32.2                                                                          •33.4                                                                         •83.1                                                                         •87.7                                                                        --    •                         7      11       •38.7                                                                          •45.2                                                                         •86.8                                                                         •88.6                                                                        --    •                         7      12       •45.8                                                                          •54.2                                                                         •88.6                                                                         •89.3                                                                        --    •                         8      3        •71.5                                                                          --    --    --   --    •                         8      4        •47.3                                                                          --    --    •57.1                                                                        •61.1                                                                         •                         8      5        •44.0                                                                          --    --    •62.4                                                                        --    •                         8      6        •44.4                                                                          --    --    •76.0                                                                              •                         8      7        •45.1                                                                          --    •(39.0)                                                                       •77.5                                                                              •                         8      8        •38.4                                                                          --    •54.8                                                                         •83.6                                                                        --    •                         8      9        •40.0                                                                          --    •76.0                                                                         •84.6                                                                        --    •                         8      10       •40.3                                                                          •(32.8)                                                                       •83.8                                                                         •88.4                                                                        --    •                         8      11       •55.6                                                                          --    •87.2                                                                         --   --    •                         8      12       •40.5                                                                          •57.8                                                                         •88.7                                                                         --   --    •                         8      13       •54.1                                                                          •62.2                                                                         •88.2                                                                         --   --    •                         9      2        •81.0                                                                          --    --    --   •(64.7)                                                                       •                         9      6        •45.0                                                                          --    --    •75.8                                                                        --    •                         9      7        •38.5                                                                          --    --    •77.4                                                                        --    •                         9      8        •35.5                                                                          --    •53.0                                                                         •84.5                                                                        --    •                         9      9        •39.0                                                                          --    •73.9                                                                         •84.9                                                                        --    •                         9      10       •37.0                                                                          •(31.7)                                                                       •83.0                                                                         •81.7                                                                        --    •                         9      11       •45.0                                                                          •46.2                                                                         •87.0                                                                         --   --    •                         9      12       •46.5                                                                          •59.3                                                                         •88.9                                                                         --   --    •                         9      13       •52.0                                                                          •65.3                                                                         •88.8                                                                         --   --    •                         10     5        •53.5                                                                          --    --    •59.6                                                                        •60.0                                                                         •                         10     6        •45.2                                                                          --    --    •74.0                                                                        --    •                         10     7        •44.0                                                                          --    --    •75.6                                                                        --    •                         10     8        •31.7                                                                          --    •43.0                                                                         •82.8                                                                        --    •                         10     9        •46.6                                                                          --    •71.4                                                                         •83.8                                                                        --    •                         10     10       •42.0                                                                          --    •80.9                                                                         •87.0                                                                        --    •                         10     11       •51.4                                                                          •(47.0)                                                                       •86.5                                                                         --   --    •                         10     12       •51.7                                                                          •59.4                                                                         •88.2                                                                         --   --    •                         ______________________________________                                    

The compound represented by the formula (I) in the liquid crystalcomposition of the present invention can be prepared by the followingmanufacturing process A.

Manufacturing Process A ##STR14## wherein R is an alkyl group such asmethyl or ethyl, and X is an elimination group such as chlorine,bromine, iodine, a p-toluenesulfonyloxy group, a benzenesulfonyloxygroup or a methanesulfonyl group.

That is, p-alkylbenzamidine hydrochloride (1) is reacted withmethoxymalonic diester in the presence of a sodium alcoholate in orderto form a diol (2), and the latter is then halogenated (3) with ahalogenating agent such as phosphorus oxychloride. Afterward,dehalogenation is carried out in the presence of a base to form adehalogenated compound (4), and the latter is then thermally treated indiethylene glycol in the presence of an alkali to obtain a compound (5).Furthermore, the latter compound is then subjected to etherification inorder to prepare the desired compound (I).

Alternatively, the compound of the formula (I) may be also prepared bythe following manufacturing process B.

Manufacturing Process B ##STR15##

That is, a compound (6) described in Collection Czechoslov. Chem.Commun., 38, p 1168 (1973) is reacted with p-alkylbenzamidinehydrochloride (1) in the presence of a sodium alcoholate in order toform a compound (7), and the latter is then subjected to an alkalitreatment to obtain a compound (5). The subsequent procedure is made inthe same manner as in the above-mentioned manufacturing process A. Thep-alkylbenzamidine hydrochloride which is the starting material iseasily available and can be prepared from a p-alkylbenzonitrile by aknown process.

The ferroelectric liquid crystal composition of the present inventionhas very excellent quick response properties, and when this compositionis used, switching elements having quick response properties can beprovided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a phase when a composition A in Example 1 is mixed with acompound represented by the formula (a).

PREFERRED EMBODIMENTS TO PRACTICE THE INVENTION

The present invention will be described in detail with reference to thefollowing examples , but it should not be limited to these examples.

EXAMPLE 1

A composition A comprising the following components represented by theformula (I) was prepared, and its physical properties were thenmeasured. Components: ##STR16##

The phase transition temperature of the composition A was ##STR17## FIG.1 shows a phase when the composition A was mixed with a compoundrepresented by the formula (II) ##STR18##

As exhibited in FIG. 1, an S_(I) * phase appears on a lower temperatureside of an S_(C) * phase when the concentration of the compound (a) is40% or more. In order that the S_(C) * phase is caused to appear at roomtemperatures, it is desirable that the concentration of the compound (a)is less than 40% or less.

Additionally, at various concentrations of the materials, response times(τ/μs) at 25° C. and spontaneous polarization values (Ps/nCcm⁻²) are asfollows:

    ______________________________________                                                            τ/μs                                                                        Ps/nCcm.sup.-2                                       ______________________________________                                        Composition A                                                                              90% by weight                                                                              80     7.0                                          Compound (a) 10% by weight                                                    Composition A                                                                              80% by weight                                                                              40     14.5                                         Compound (a) 20% by weight                                                    Composition A                                                                              70% by weight                                                                              35     25.0                                         Compound (a) 30% by weight                                                    ______________________________________                                    

As described above, when the achiral compound represented by the formula(I) is mixed with the optically active compound represented by theformula (II), the ferroelectric liquid crystal composition can beobtained which has the high upper limit temperature of the S_(C) * phaseand excellent response properties.

Here, the phase transition temperature was measured from the observationof texture by means of DSC and a polarizing microscope. The responsetime was measured as follows: A liquid crystal element was first putbetween two polarizing plates perpendicular to each other, and arectangular wave of ±10 V/μm and 1 KHz was then applied thereto, so thatthe strength of transmitted light changed. The response time wasdetermined by this change. Furthermore, the spontaneous polarizationvalues were measured in accordance with the Sowyer-Tower method. In theundermentioned examples, there were shown values under the sameconditions.

EXAMPLE 2

A composition B was prepared by blending the composition A with thefollowing compounds represented by the formula (III), and its physicalproperties were then measured. ##STR19##

A phase transition temperature of the composition B was ##STR20##Furthermore, a response time at 25° C. was 28 microseconds.

In view of the fact that most of the conventional known ferroelectricliquid crystal compositions have a response time of 100 microseconds, itis definite that the ferroelectric liquid crystal composition of thepresent invention has very quick response properties.

EXAMPLE 3

A composition C was prepared from the following components representedby the formulae (I) and (II), and its physical properties were measured.##STR21##

The phase transition temperature of the composition C was ##STR22## Inthe composition C, a response time (τ) at 25° C. was 43 microseconds, aspontaneous polarization value (Ps) was 19.2 nCcm⁻², and a tilt anglewas 22.5°.

EXAMPLE 4

A composition D having the following components was prepared from thecompounds represented by the formulae (I) and (II), and its physicalproperties were measured. ##STR23##

The phase transition temperature of the composition D was ##STR24## Inthe composition D, a response time (τ) at 25° C. was 45 microseconds, aspontaneous polarization value (Ps) was 15.0 nCcm⁻², and a tilt anglewas 20.9°.

EXAMPLE 5

A composition E was prepared from the following components representedby the formulae (I) and (II), and its physical properties were thenmeasured. ##STR25##

The phase transition temperature of the composition E was ##STR26## Inthe composition E, a response time (τ) at 25° C. was 41 microseconds, aspontaneous polarization value was 14.0 nCcm⁻², and a tilt was 20.8°.

EXAMPLE 6

A composition F comprising the following components represented by theformulae (I), (II) and (III) was prepared, and its physical propertieswere then measured. ##STR27##

The phase transition temperature of the composition F was ##STR28## Inthe composition F, a response time (τ) at 25° C. was 23 microseconds, aspontaneous polarization value was 57.1 nCcm⁻², and a tilt angle was20°.

EXAMPLE 7

A composition G comprising the following components represented by theformulae (I), (II) and (III) was prepared, and its physical propertieswere then measured. ##STR29##

The phase transition temperature of the composition G was ##STR30## Inthe composition G, a response time (τ) at 25° C. was 33 microseconds, aspontaneous polarization value was 87.5 nCcm⁻², and a tilt angle was23.7° , and a response time (τ) at 30° C. was 28 microseconds, aspontaneous polarization value was 81.4 nCcm⁻², and a tilt angle was23.6°.

EXAMPLE 8

A composition H comprising the following components represented by theformulae (I), (II) and (III) was prepared, and its physical propertieswere then measured. ##STR31##

The phase transition temperature of the composition H was ##STR32## Inthe composition H, a response time (τ) at 25° C. was 25 microseconds, aspontaneous polarization value was 59.9 nCcm⁻², and a tilt angle was22.1°.

EXAMPLE 9

A composition I comprising the following components represented by theformulae (I), (II) and (III) was prepared, and its physical propertieswere then measured. ##STR33##

The phase transition temperature of the composition I was ##STR34## Inthe composition I, a response time (τ) at 25° C. was 24 microseconds, aspontaneous polarization value was 66.9 nCcm⁻², and a tilt angle was22.1°.

EXAMPLE 10

A composition J comprising the following components represented by theformulae (I), (II) and (III) was prepared, and its physical propertieswere then measured. ##STR35##

The phase transition temperature of the composition J was ##STR36## Inthe composition J, a response time (τ) at 25° C. was 24 microseconds, aspontaneous polarization value was 57.0 nCcm⁻², and a tilt angle was22.3°.

POSSIBILITY OF INDUSTRIAL UTILIZATION

The composition of the present invention can exhibit an S_(C) * phase inan extensive temperature range inclusive of room temperatures and hasquick response properties, and therefore it can solve the problem ofconventional liquid crystal elements which has been outstanding for along time. Moreover, by the use of this liquid crystal composition,switching elements having quick response properties can be obtained.

We claim:
 1. A ferroelectric liquid crystal composition comprising atleast one achiral compound represented by formula (I) ##STR37## whereineach of R¹ and R² is an alkyl group having 1 to 18 carbon atoms, and atleast one optically active compound represented by formula (II)##STR38## wherein R³ is an alkyl group or an alkoxy group having 1 to 18carbon atoms, R⁴ is an alkyl group having 2 to 18 carbon atoms, X is asingle bond, --COO--, --OCO--, --N═CH--, --CH═N--, --OCH₂ -- or --CH₂O--, each of m and n is an integer of 1 or 2, and the symbol * is anasymmetric carbon atom and at least one compound represented by formula(III) ##STR39## wherein each of ##STR40## R⁵ is an alkyl group or analkoxy group having 1 to 18 carbons atoms, Y is a single bond, --CH₂ O--or --OCH₂ --, p is an integer of 0 or 1, and Z is an alkyl group having1 to 18 carbon atoms or --C*H(CH₃)--O--R⁶ wherein R⁶ is an alkyl grouphaving 1 to 18 carbon atoms and the symbol * represents an asymmetriccarbon atom.
 2. A ferroelectric liquid crystal composition according toclaim 1 wherein the proportion of the compound represented by formula(I) is from 20 to 98% by weight, that of the compound represented byformula (II) is from 1 to 40% by weight and that of the compoundrepresented by formula (III) is from 1 to 40%.
 3. A ferroelectric liquidcrystal composition according to claim 1 wherein the compoundrepresented by the formula (I) comprises at least one compound whereinR¹=n-C_(x) H_(2x+1) and R² =n-C_(y) H_(2y+1) and wherein when x=6, y=6 to15, when x=7, y=5 to 12, when x=8, y=3 to 13, when x=9, y=2 or 6 to 13,or when x=10, y=5 to
 12. 4. A ferroelectric liquid crystal compositionaccording to claim 1 wherein the compound represented by formula (II)comprises at least one compound represented by the formulae ##STR41##wherein R³ is an alkyl group or an alkoxy group having 1 to 18 carbonatoms, and R⁴ is an alkyl group having 2 to 18 carbon atoms.
 5. Aferroelectric liquid crystal composition according to claim 1 whereinthe compound represented by the formula (III) comprises at least one ofthe compounds represented by the formulae ##STR42## wherein R⁵ is analkyl group or an alkoxy group having 1 to 18 carbon atoms, and R⁶ is analkyl group having 1 to 18 carbon atoms.
 6. A ferroelectric liquidcrystal composition according to claim 1 wherein the compoundrepresented by the formula (III) comprises at least one of ##STR43## or4'-heptyl-4-[4-{2-(butanoyloxy)-propoxy}-phenoxymethyl]-biphenyl or4'-nonyl-4-[4-{2-(pentanoyloxy)-propoxy}-phenoxymethyl]-biphenyl.
 7. Aferroelectric liquid crystal composition according to claim 1 whereinthe compound represented by formula II comprises at least one of thecompounds represented by the formulae ##STR44##
 8. A light switchingelement comprising a ferroelectric liquid crystal composition whichcomprises at least one achiral compound represented by formula (I)##STR45## wherein each of R¹ and R² is an alkyl group having 1 to 18carbon atoms;at least one optically active compound represented byformula (II) ##STR46## wherein R³ is an alkyl group or an alkoxy grouphaving 1 to 18 carbon atoms, R⁴ is an alkyl group having 2 to 18 carbonatoms, X is a single bond, --COO--, --OCO--, --N═CH--, --CH═N--, --OCH₂-- or --CH₂ O--, each of m and n is an integer of 1 or 2, and thesymbol * indicates an asymmetric carbon atom; and at least one compoundrepresented by formula (III) ##STR47## wherein each of ##STR48## R⁵ isan alkyl group or an alkoxy group having 1 to 18 carbon atoms, Y is asingle bond, --CH₂ O-- or --OCH₂ --, p is an integer of 0 or 1, and Z isan alkyl group having 1 to 18 carbon atoms or --C*H(CH₃)--O--R⁶ is analkyl group having 1 to 18 carbon atoms and the symbol * represents anasymmetric carbon atom.
 9. A light switching element according to claim8 wherein the proportion of the compound represented by formula (I) isfrom 20 to 98% by weight, that of the compound represented by formula(II) is from 1 to 40% by weight and that of the compound represented byformula (III) is from 1 to 40% by weight.
 10. A light switching elementwhich includes a ferroelectric liquid crystal composition according toclaim 8, wherein the compound represented by the formula (I) comprisesat least one compound whereinR¹ =n-CxH₂ x+1 and R₂ =n-CyH₂ y+1 whereinwhen x=6, y=6 to 15, when x=7, y=5 to 12, when x=8, y=3 to 13, when x=9,y=2 or 6 to 13, or when x=10, y=5 to
 12. 11. A light switching elementwhich includes a ferroelectric liquid crystal composition according toclaim 8, wherein the composition represented by formula (II) comprisesat least one compounds represented by the formulae ##STR49## wherein R³is an alkyl group or an alkoxy group having 1 to 18 carbon atoms, and R⁴is an alkyl group having 2 to 18 carbon atoms.
 12. A light switchingelement which includes ferroelectric liquid crystal compositionaccording to claim 8 wherein the compound represented by formula IIcomprises at least one of the compounds represented by the formulae##STR50##
 13. A light switching element which includes a ferroelectricliquid crystal composition according to claim 8 wherein the compoundrepresented by the formula (III) comprises at least one of the compoundsrepresented by the formulae ##STR51## wherein R⁵ is an alkyl group or analkoxy group having 1 to 18 carbon atoms, and R⁶ is an alkyl grouphaving 1 to 18 carbon atoms.
 14. A light switching element whichincludes a ferroelectric liquid crystal composition according to claim 8wherein the compound represented by the formula (III) comprises at leastone of ##STR52## or4'-heptyl-4-[4-{2-(butanoyloxy)-propoxy}-phenoxymethyl]-biphenyl or4'-nonyl-4-[4-{2-(pentanoyloxy)-propoxy}-phenoxymethyl]-biphenyl.